Add an API example in Go to measure trip times with different

percentages of people cancelled.

book/src/dev/api.md

@@ -4,12 +4,16 @@ Suppose you're tired of manually fiddling with traffic signals, and you want to
 use machine learning to do it. You can run A/B Street without graphics and
 automatically control it through an API.
 
-## Example
+## Examples
 
 This
 [Python example](https://github.com/dabreegster/abstreet/blob/master/headless/examples/python_client.py)
 has everything you need to get started.
 
+Also check out the
+[Go example](https://github.com/dabreegster/abstreet/blob/master/headless/examples/go_client.go),
+which demonstrates just a few of the API calls.
+
 ## API details
 
 > **Under construction**: The API will keep changing. There are no backwards

headless/examples/go_client.go

@@ -0,0 +1,118 @@
+/*
+This example runs the same scenario repeatedly, each time cancelling a
+different number of trips uniformly at random. The eventual goal is to quantify
+how many trips need to be cancelled to substantially speed up remaining ones.
+
+Before running this script, start the API server:
+
+> cargo run --release --bin headless -- --port=1234 data/system/scenarios/montlake/weekday.bin
+*/
+
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+)
+
+const (
+	api             = "http://localhost:1234/"
+	mapName         = "montlake"
+	hoursToSimulate = 24
+)
+
+func main() {
+	for pct := int64(100); pct >= 0; pct -= 10 {
+		if err := run(pct); err != nil {
+			fmt.Println("Failure:", err)
+			break
+		}
+	}
+}
+
+func run(pct int64) error {
+	_, err := post("sim/load", SimFlags{
+		Load:      fmt.Sprintf("data/system/scenarios/%v/weekday.bin", mapName),
+		Modifiers: []ScenarioModifier{{CancelPeople: pct}},
+	})
+	if err != nil {
+		return err
+	}
+
+	_, err = get(fmt.Sprintf("sim/goto-time?t=%v:00:00", hoursToSimulate))
+	if err != nil {
+		return err
+	}
+
+	resp, err := get("data/get-finished-trips")
+	if err != nil {
+		return err
+	}
+	var trips FinishedTrips
+	if err := json.Unmarshal([]byte(resp), &trips); err != nil {
+		return err
+	}
+
+	numAborted := 0
+	numSucceeded := 0
+	totalDuration := 0.0
+	for _, trip := range trips.Trips {
+		if trip[2] == nil {
+			numAborted++
+		} else {
+			numSucceeded++
+			totalDuration += trip[3].(float64)
+		}
+	}
+
+	fmt.Printf("%v with %v%% of people cancelled: %v trips aborted, %v trips succeeded totaling %v seconds\n", mapName, pct, numAborted, numSucceeded, totalDuration)
+
+	return nil
+}
+
+func get(url string) (string, error) {
+	resp, err := http.Get(api + url)
+	if err != nil {
+		return "", err
+	}
+	body, err := ioutil.ReadAll(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return "", err
+	}
+	return string(body), nil
+}
+
+func post(url string, body interface{}) (string, error) {
+	encoded, err := json.Marshal(body)
+	if err != nil {
+		return "", err
+	}
+	resp, err := http.Post(api+url, "application/json", bytes.NewReader(encoded))
+	if err != nil {
+		return "", err
+	}
+	respBody, err := ioutil.ReadAll(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return "", err
+	}
+	return string(respBody), nil
+}
+
+type SimFlags struct {
+	Load      string             `json:"load"`
+	Modifiers []ScenarioModifier `json:"modifiers"`
+}
+
+type ScenarioModifier struct {
+	CancelPeople int64
+}
+
+type FinishedTrips struct {
+	// Vec<(Time, TripID, Option<TripMode>, Duration)>
+	Trips [][]interface{} `json:"trips"`
+}